Regulations concerning exhaust amount of toxic substances contained in automotive exhaust gas have been increasingly tightened. In the United States, it is said that an onboard particulate matter diagnostic system will be essential in 2010, for example.
It has been reported that a gasoline-powered vehicle that employs a direct injection system discharges particulate matter (PM) in an amount larger than that of a diesel-powered vehicle provided with a diesel particulate filter (DPF).
Patent Document 1 focuses on the fact that particulate matter contained in exhaust gas consists of conductive particles, and discloses a particulate matter detection element and a particulate matter detection filter that utilize a phenomenon in which the electrical resistance of an electrical insulating member decreases when conductive particulate matter adheres to the electrical insulating member.
Patent Document 2 discloses technology that receives light that has been emitted from a light source and passed through an exhaust gas flow using a light-receiving section, detects the opacity of the exhaust gas that has a constant correlation function with the particulate matter concentration, and converts the detected opacity into the particulate matter concentration using a control means.    Patent Document 1: JP-A-59-060018    Patent Document 2: JP-A-04-203413